This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Quarantining new fish is a good habit for all aquaria, but is not
absolutely necessary for success. Quarantining is simply keeping a
fish in a separate tank for long enough to be certain that it is
disease free. Many beginners do fine without a quarantine tank, and
object to the cost of another setup. A quarantine tank does cost more,
but if a hobbyist has hundreds of dollars invested in fish, it is
cheaper to have a separate quarantine tank than to replace fish killed
by a newly introduced disease. Also, many of us become attached to
fish and do not want to expose our pets to diseases from newcomers, no
matter what the cost.
The purpose of quarantining is to avoid introducing new diseases to a
stable system, and to be able to better observe new fish for signs of
disease. A quarantine tank can also double as a hospital tank for sick
fish. Hospital tanks are good because they lower the cost of using
medicines and keep diseased fish separate from healthy ones.
Quarantine is probably most important for saltwater tanks/reef systems
because of the difficulty of treating diseases, or wild-caught
freshwater fish because they are probably not disease-free.
Quarantining itself can stress fish so be sure quarantine is as
stress-free as possible.
To set up a quarantine or hospital tank:
* Keep an extra filter -- a sponge filter is ideal -- or piece of
filter floss in an established tank, so that you don't have to
keep the quarantine tank set up at all times. Some people choose
instead to keep the filter going with guppies or danios (for
freshwater) or mollies (for saltwater).
* If you don't keep the tank running, use old tank water to fill the
tank. So: old tank water + established filter = instant
* Add a spare airpump and heater. If you haven't messed with the
heater during storage, it should come to wherever you had it last
* Consider using Amquel or equivalent when medicating the tank in
case the biological filter bacteria are sensitive to the
medication. Sick fish are especially susceptible to ammonia. (Note
that ammonia which has been bound with Amquel still shows up on a
nessler ammonia test. So, if you are planning on testing for
ammonia in that tank, you need to use a salicylate ammonia test.)
* For a hospital tank, do small, frequent water changes (even every
If possible, quarantine all of your new fish for about three weeks.
During that time, gradually acclimate the fish to your tank's
parameters: hardness, pH, salinity, temperature, etc., and watch for
and treat any signs of disease.
Do not medicate quarantined fish "just in case." Only treat evident,
definitely identified diseases. Treating all quarantined fish with a
bunch of medicines will just lead to weakened fish and antibiotic
Once you are done with the quarantine, if you treated any especially
nasty diseases, it is good to disinfect the tank and reestablish the
filter. Chlorine bleach or strong saltwater (for freshwater) work
well. Be sure all traces of bleach are rinsed off. Another good
disinfectant is potassium permanganate (Jungle's Clear Water is one
commercial way to get it).
If you choose not to quarantine, do not add store water to your tank
with the new fish (see the BEGINNER FAQ for acclimation ideas).